1. Field of the Invention
The present invention relates to a nonaqueous electrolyte battery.
2. Description of the Related Art
A separator for nonaqueous electrolyte battery such as lithium ion secondary battery needs to not only prevent shortcircuit due to contact of positive electrode with negative electrode but also retain the electrolyte so that lithium ion can rapidly move through the electrolyte. The material of the separator needs to be not only chemically stable to organic solvents but also electrochemically stable.
Therefore, as the separator for nonaqueous electrolyte battery which is commercially available at present, there is used a porous membrane made of a polyolefin such as polyethylene and polypropylene. In such a polyolefin-based membrane, the polyolefin portion has no lithium ionic conductivity.
The production of a porous separator is carried out mainly by stretching method or wet method. The stretching method is a method for the production of a separator which comprises stretching a polymer to form directional pores in a membrane (U.S. Pat. No. 3,953,566, etc.). The wet method is a method for the production of a separator having a nondirectional network structure which comprises spreading a polymer paste to form a sheet, and then dipping the polymer sheet in a solution so that the solvent in which the polymer is dissolved is removed to form pores (GB 2,026,381).
Thereafter, the use of a solid polymer electrolyte comprising a mixture of a polyethylene oxide and a lithium salt instead of organic electrolyte as a separator has been proposed. However, such a solid polymer electrolyte has a low ionic conductivity at room temperature and thus was not put in practical use.
Then, a porous polymer electrolyte has been developed as a separator. The porous polymer electrolyte comprises a polymer which can be swollen with an electrolyte such as polyvinylidene fluoride instead of polyolefin having no lithium ionic conductivity. The porous polymer electrolyte retains an electrolyte in the pores. The polymer portion which has been swollen with the electrolyte, also, exhibits lithium ionic conductivity. A lithium ion secondary battery which comprises such a porous polymer electrolyte to improve the discharge performance at a high rate has been studied as well (JP-A-8-195220 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)).
Among the methods for the production of a porous polyolefin-based separator, the wet method is applied to the preparation of a porous polymer comprising a polyvinylidene fluoride (JP-A-9-259923).
When such a porous polymer electrolyte is applied to lithium ion secondary battery, lithium ions can move into the electrolyte presented in the pores of the porous polymer electrolyte. Thus, it is expected that the high rate discharge performance of the battery can be improved.
In a nonaqueous electrolyte battery such as lithium ion secondary battery, the majority of lithium ion participating in the electrode reaction in the charge-discharge reaction is not from the amount of lithium ion dissolved in the electrolyte but from the one which is contained in the active material of an electrode. Lithium ion participated in the reaction is extracted. Namely, the lithium ion moves from the electrode through the electrolyte to the opposite electrode. Therefore, the reduction of the thickness of the separator or the control over the porosity of the separator is very important for the improvement of high rate discharge performance.
However, with the porous separator prepared by the stretching method it is difficult to reduce its thickness to about 25 xcexcm or less and control its porosity. Therefore, it has been very difficult to prepare a nonaqueous electrolyte battery such as lithium ion secondary battery having excellent high rate discharge performance.
Further, the porous separator prepared by the wet method is disadvantageous in that the use of a large amount of an organic solvent requires a complicated production process that adds to the production cost. Moreover, the wet method is disadvantageous in that an anisotropy can easily take place in the direction of extraction of the solvent dissolving the polymer therein from the polymer paste, making it difficult to make the pore distribution uniform. Therefore, a nonaqueous electrolyte battery comprising a porous separator prepared by the wet method has a nonuniform current distribution. Thus, it is impossible to obtain sufficient high rate discharge performance and safety.
A technique has been reported, which comprises providing a porous material formed by insulating polymer particles between the positive electrode and the negative electrode as a separator to solve the foregoing problems with nonaqueous electrolytic batteries (JP-A-1-167948). A nonaqueous electrolyte battery comprising such a porous material exhibits good performance, because lithium ions can move through the electrolyte presented in three-dimensional continuous pores among the polymer particles. JP-A-1-167948 discloses a nonaqueous electrolyte battery obtained by winding a porous membrane comprising spherical resin particles bonded to each other with positive and negative electrodes.
Further, JP-A-11-102730 discloses a nonaqueous electrolyte battery comprising an insulating layer-integrated positive electrode and/or negative electrode obtained by attaching a porous resin layer formed by a particulate insulating resin to the surface of a positive electrode and/or negative electrode.
In the foregoing arrangement, this resin layer is provided to make up for the deficiency in the shutdown mechanism of the separator. A nonaqueous electrolyte battery comprising such an electrode and a porous polypropylene separator in combination has been disclosed.
However, when the polymer particles constituting the porous resin layer has no ionic conductivity, the moving path of lithium ion in the porous resin layer is increased. The curvature of the moving path of lithium ion at the border of the porous resin layer with the electrode is increased. Accordingly, the current distribution is nonuniform and hence the high rate discharge performance thereof is remarkably deteriorated.
It is an object of the present invention to obtain a nonaqueous electrolyte battery having excellent high rate discharge performance and a high safety.
According to the present invention, a nonaqueous electrolyte battery comprises an ion-conductive polymer particles provided between a positive electrode and a negative electrode, the positive electrode and the negative electrode being insulated from each other by the polymer particles.
In the foregoing nonaqueous electrolyte battery, three-dimensional continuous pores are preferably formed among the ion-conductive polymer particles, and its three-dimensional continuous pores is possible to contain a nonaqueous electrolyte. The ion-conductive polymer particles are wetted or swollen with the nonaqueous electrolyte.
In the foregoing nonaqueous electrolyte battery, the ion-conductive polymer particles have micro pores which preferably contain the nonaqueous electrolyte.
In the foregoing nonaqueous electrolyte battery, the ion-conductive polymer particles are preferably fixed to a surface of at least one of the positive electrode and the negative electrode.
In the foregoing nonaqueous electrolyte battery, the shape of the ion-conductive polymer particles is preferably sphere or the like.
In the foregoing nonaqueous electrolyte battery, the average particle size of the ion-conductive polymer particles is preferably not more than that of the active material particles of at least one of the positive electrode and the negative electrode.
In the foregoing nonaqueous electrolyte battery, the ion-conductive polymer particles preferably contain an elastomer.